This application relates to a unique control and method for correcting errors in at least two different variables in a refrigerant system, wherein each of two error correction algorithms take into account an error signal from both variables.
Refrigerant systems typically include a compressor for compressing a refrigerant and a first heat exchanger receiving the refrigerant. Downstream of the first heat exchanger is an expansion device that expands the refrigerant. Adjustable expansion devices are known, that can be opened to varying amounts to change refrigerant pressure throughout the system. From the expansion device, the refrigerant travels to a second heat exchanger and then back to the compressor.
A control for the system is operable to take in inputs, such as a user demand for a particular hot water temperature at a faucet. Other applications may be to achieve a temperature in an environment conditioned by the refrigerant cycle (air conditioning or heat pump), and control aspects of the refrigerant cycle to achieve the demanded temperature. As an example, in one use of a refrigerant cycle, the first heat exchanger is utilized to heat water. Among the uses may be a hot water system for heating water.
In a hot water system, a control takes in an operator demand for a particular water temperature, and controls the amount of water flowing through the first heat exchanger. The more water that flows through the first heat exchanger, the lesser the temperature of the hot water at the outlet. Thus, to achieve higher temperatures for the water, the volume of water flow is reduced.
Further, and again to achieve the desired temperature demanded for the hot water, the refrigerant circuit must be controlled to provide sufficient heat at the first heat exchanger to heat the water to the desired temperature. Thus, two aspects of the hot water circuit must be controlled; the amount of water flowing through the first heat exchanger, and also an aspect of the refrigerant cycle. The amount of water delivered to the first heat exchanger can be controlled by controlling the speed of the water pump. In one embodiment, the refrigerant cycle is controlled in a system developed by the assignee of this application, by controlling the expansion device to provide a desired discharge pressure. Of course, a desired temperature or other condition could also be provided.
The controls for controlling the two variables, e.g., water pump speed and expansion device opening, typically have each taken into account an error, an integral of that error, and a derivative of this error. Such controllers are known as PID controllers. This type of controller is quite useful in controlling a condition, and providing feedback to adjust the condition such that it remains as desired. Such controls are known as single input, single output or “SISO.” These systems are somewhat sensitive to variations in some of the variables surrounding the system, as examples, ambient air temperature, the temperature of the water entering the heat exchanger, etc.
There is another deficiency in the above described control when the two control variables have an impact on each other. That is, as one of the variables, e.g., water pump speed, changes, it will change the pressure, yet the error correction algorithm for one variable does not anticipate the impact changes to the other will cause, until after the change has occurred. Thus, the efficiency of the overall system may not be as high as would be desired, in that each of the two variables would be sensitive to change in the other.